It is known that lactic acid bacteria such as bacteria of the genus Lactococcus or bacteria of the genus Bifidobacterium (hereinafter, abbreviated to “Bifidobacterium”) have an intestinal function-regulating activity, immuno-stimulating activity, and anti-cancer activity. Accordingly, lactic acid bacteria tend to be often formulated in food in accordance with an increase of health consciousness of consumers. In particular, Bifidobacterium such as Bifidobacterium longum is one of the predominant bacterial strains in intestinal microflora formed in the human intestinal tract, and demands for fermented milk or other food products containing viable Bifidobacterium are increasing.
Bifidobacterium exhibits a poor proliferation potency in milk medium. Accordingly, various growth-stimulating substances are generally formulated in fermented milk so that Bifidobacterium is contained therein at a constant content, such as 1×107 CFU/ml. However, the growth-stimulating substances are generally expensive and may deteriorate taste. In addition, preservation of Bifidobacterium under acidic conditions is difficult and tends to result in death thereof. Thus, the viable count of Bifidobacterium in fermented milk products decreases with accelerating speed during distribution of the fermented milk products.
Accordingly, it is expected that a growth promotion of Bifidobacterium or an improvement of survivability thereof during storage will enable not only preparation of fermented milk containing a large amount of viable Bifidobacterium, but also preparation of fermented milk keeping an abundant amount of viable Bifidobacterium from immediately after preparation until being consumed.
Various methods for promoting growth of Bifidobacterium or improving the survivability thereof during storage by fermentation with Bifidobacterium and another lactic acid bacterium without adding any growth-stimulating substances or the like have been disclosed. For example, (1) yogurt containing Lactococcus lactis subsp. lactis, Lactococcus lactis subsp. cremoris, and Bifidobacterium, and a method for preparing the yogurt has been disclosed (see, for example, Patent Document 1), as a method for promoting growth of Bifidobacterium to prepare fermented milk.
For example, (2) a method for fermenting milk with Bifidobacterium, including cultivating Bifidobacterium breve together with Lactococcus lactis subsp. lactis, which forms neither diacetyl nor acetoin, on a medium containing milk as the main component thereof has been disclosed (see, for example, Patent Document 2), as a method for improving the survivability of Bifidobacterium during storage of fermented milk.    Patent Document 1: Japanese Patent Publication No. 3,364,491.    Patent Document 2: Japanese Patent Publication No. 3,068,484.